Handbook of Photovoltaic Science and Engineering

Antonio Luque, Steven Hegedus, Antonio Luque, Steven Hegedus

• Publisher: Wiley
• Year: 2011
• ISBN:
  9780470976128
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

The most comprehensive, authoritative and widely cited reference on photovoltaic solar energy

Fully revised and updated, the Handbook of Photovoltaic Science and Engineering, Second Edition incorporates the substantial technological advances and research developments in photovoltaics since its previous release. All topics relating to the photovoltaic (PV) industry are discussed with contributions by distinguished international experts in the field.

Significant new coverage includes:

• three completely new chapters and six chapters with new authors
• device structures, processing, and manufacturing options for the three major thin film PV technologies
• high performance approaches for multijunction, concentrator, and space applications
• new types of organic polymer and dye-sensitized solar cells
• economic analysis of various policy options to stimulate PV growth including effect of public and private investment

Detailed treatment covers:

• scientific basis of the photovoltaic effect and solar cell operation
• the production of solar silicon and of silicon-based solar cells and modules
• how choice of semiconductor materials and their production influence costs and performance
• making measurements on solar cells and modules and how to relate results under standardised test conditions to real outdoor performance
• photovoltaic system installation and operation of components such as inverters and batteries.
• architectural applications of building-integrated PV

Each chapter is structured to be partially accessible to beginners while providing detailed information of the physics and technology for experts. Encompassing a review of past work and the fundamentals in solar electric science, this is a leading reference and invaluable resource for all practitioners, consultants, researchers and students in the PV industry.

Chapter 1

Achievements and Challenges of Solar Electricity from Photovoltaics

Steven Hegedus1 and Antonio Luque2

1Institute of Energy Conversion, University of Delaware, USA

2Instituto de Energía Solar, Universidad Politécnica de Madrid, Spain

1.1 The Big Picture

Congratulations! You are reading a book about a technology that has changed the way we think about energy. Photovoltaics (or PV) is an empowering technology that has shown that it can generate electricity for the human race for a wide range of applications, scales, climates, and geographic locations. Photovoltaics can bring electricity to a rural homemaker who lives 100 kilometers and 100 years away from the nearest electric grid connection in her country, thus allowing her family to have clean, electric lights instead of kerosene lamps, to listen to a radio, and to run a sewing machine for additional income. It can pump clean water from underground aquifers for drinking or watering crops or cattle. Or, photovoltaics can provide electricity to remote transmitter stations in the mountains, allowing better communication without building a road to deliver diesel fuel for its generator. It can allow a suburban or urban homeowner to produce some or all of their annual electricity, selling any excess solar electricity back into the grid. It can help a major electric utility in Los Angeles, Tokyo, or Madrid to meet its peak load on hot summer afternoons when air conditioners are working full time. Finally, photovoltaics has been powering satellites orbiting the Earth for 30 years or vehicles roving over the surface of Mars.

Every day the human race is more aware of the need for sustainable management of its Planet Earth. It upholds almost seven billion human beings of which one billion have adopted a high-consumption lifestyle which is not sustainable. “High consumption” used to refer materials that could become scarce, but it increasingly refers to energy. Here the term energy, refers to “useful energy” (or exergy), that once used is degraded, typically to waste heat, and will be no longer be useful.

Here we are concerned with electrical energy which is a secondary form of energy. Fossil fuel (coal, petroleum and natural gas) combustion and nuclear fission are the primary processes which create heat to turn water into steam which rotate giant turbines which generate electricity. When the C–H bonds in fossil fuels are burned in the presence of air for heat, they produce CO2, and H2O. The latter waste is not a problem because there is already so much water in the seas and in the atmosphere. But CO2 is a different story. Analysis of the air bubbles embedded in Antarctic ice layers provides information on the CO2 concentration of the atmosphere in the last 150 000 years. This content shows an unprecedented growth in the last 300 years, coinciding with the beginning of the industrialization. This fact is linked by most scientists to global climate change, including global warming, sea level rise, more violent storms, and changes in rainfall. This will disrupt agriculture, disease control, and other human activities. Thus, a substantial fraction of our energy must be generated without any C emissions within the next 10–20 years, or else the Earth will become a dangerous experiment. Besides, fossil fuels cannot last forever. Supplies of petroleum and natural gas will both peak and then decrease within decades if not years, and coal within few centuries. We must develop large-scale alternatives to burning fossil fuels very soon.

Another primary energy source for electrical generation is radioactivity in the form of uranium, which when conveniently transformed, fuels nuclear plants through nuclear fission. Concerning the uranium, most of it consists of the 238 isotope, which is not “fissile” (not a nuclear fuel) and only about 0.7% is the 235 isotope, which is fissile. With the present technology, nuclear fuel will peak within decades. However, uranium 238 can be converted to an artificial fissile fuel by proper bombardment with neutrons. With this technology, not fully commercial today, it would be possible to have nuclear power (with unproven cost effectiveness) maybe for a millennium. Nuclear fusion, which is a totally different nuclear technology, could be practically inexhaustible, but its practical feasibility is very far from being proven.

While nuclear plants emit no CO2, they are still inherently dangerous. Nuclear engineers and regulators take many precautions to ensure safe operation, and (excepting very few cases) the power plants function without catastrophic problems. But the storage of highly radioactive wastes, which must remain controlled for centuries, remains an unsolved issue worldwide, along with the possibility of diversion of nuclear fuel to making a bomb.

So the situation at the beginning of the 21st century is that the previous century's methods of generating our most useful form of energy, electricity, are recognized as unsustainable, due to either increasing CO2 poisoning of the atmosphere or the increasing stockpile of radioactive waste with no safe storage. What about using existing energy more efficiently? This will be crucial for slowing and perhaps even reversing the increased CO2 levels. Doing more with less energy or just doing less (considered unpopular with growth-oriented economic advocates) are certainly necessary to reduce our demand for energy. But a growing world population with a growing appetite for energy is difficult to reconcile with using less energy. Besides, there is a large group whose voices are often not heard in this discussion—namely, the one out of three human beings who lack any electricity at all.

In fact, access to and consumption of electricity is closely correlated with quality of life, up to a point. Figure 1.1 shows the human development Index (HDI) for over 60 countries, which includes over 90% of the Earth's population, versus the annual per capita electricity use (adapted from [1]). The HDI is compiled by the UN and calculated on the basis of life expectancy, educational achievement, and per capita gross domestic product. To improve the quality of life in many countries, as measured by their HDI, will require increasing their electricity consumption by factors of 10 or more, from a few hundred to a few thousand kilowatt-hours (kW h) per year.

Figure 1.1 Human development index (HDI) versus per capita kW usage in year 2000 [1]

Adding two billion more inhabitants with increasing appetites for energy to the high-consumption pattern of today's one billion in the developed World, as would be expected from the development of China and India, would lead to unbearable stresses both in materials and energy. Barring their access (and that of others) to the wealth of the Western lifestyle is unfeasible, in addition to being unethical.

Renewable energies, and in particular solar energy, are the only clear solution to these issues. As matter of fact the amount of energy arriving on Earth from the Sun is gigantic: in the range of 10 000 times the current energy consumption of the human species. The ability of various forms of renewable energy to meet the “terawatt challenge” of providing world's present demand of 13 TW has been published [2]. We can also add geothermal energy (not renewable, properly speaking) and tidal energy, but they are insignificant in global terms, although locally, in some cases, their exploitation may be attractive.

Wind is generated by the solar energy (through the differential heating of the Earth in equatorial and polar regions). It has been calculated [3] that about 1% of the solar energy (10 times the global current consumption of energy) is converted into wind, but only a 4% of this is actually usable (but still 0.4 times the current consumption). It is estimated that with aggressive exploitation, land- and water-based wind generation is capable of providing about 10% of the world's expected energy demand [2]. Biomass converts solar energy in fuels but its efficiency is also very low, and its use for food has priority. Waves are caused by the wind and therefore a small fraction of the wind energy is passed to them. Sea currents, as winds also originate in the solar energy. The fraction passing to them is uncertain, but probably small. Finally, hydropower, produced by the transport of water from the sea to the land by means of solar energy, represents a tiny fraction of the total energy income, and the most promising sites are already in use. Summarizing, the direct exploitation of the solar energy is the real big energy resource [4].

Using photovoltaics with an efficiency of 10%, solar energy can be converted directly into enough electricity to provide 1000 times the current global consumption. Restricting solar collection to the earth's solid surface (one quarter of the total surface area), we still have a potential of 250 times the current consumption. This means that using 0.4% of the land area could produce all the energy (electricity plus heat plus transportation) currently demanded. This fraction of land is much smaller than the one we use for agriculture.

Achieving the required strong penetration of solar energy is not trivial. In the rest of this chapter we shall present a description of the status PV and broadly outline some of the challenges for it to become a TW scale energy source. But let us advance arguments that are seldom spoken: (a) PV is technologically more mature than advanced nuclear fission or nuclear fusion technology, the two non-renewable CO2-free energies permitting substantial increments of the global energy production; (b) even well-developed wind energy cannot match the amount of energy directly available from the sun; (c) biomass energy can expect further scientific development, but will probably not reach efficiency levels that will make of it a global alternative to solve the issues presented; (d) concentrating solar thermal power (CSP) could produce electricity in concurrence with PV. We think that PV has a bigger innovation potential and has also modularity properties (it operates at small or large scale) and lacks the geographic limitations of CSP which makes it a clear winner in this competition.

1.2 What is Photovoltaics?

PV is the technology that generates direct current (DC) electrical power measured in watts (W) or kilowatts (kW) from semiconductors when they are illuminated by photons. As long as light is shining on the solar cell (the name for the individual PV element), it generates electrical power. When the light stops, the electricity stops. Solar cells never need recharging like a battery. Some have been in continuous outdoor operation on Earth or in space for over 30 years.

Table 1.1 lists some of the advantages and disadvantages of PV. Note, that they include both technical and nontechnical issues

Table 1.1 Advantages and disadvantages of photovoltaics

Advantages of photovoltaics

Fuel source is vast, widely accessible and essentially infinite No emissions, combustion or radioactive waste (does not contribute perceptibly to global climate change or air/water pollution) Low operating costs (no fuel) No moving parts (no wear); theoretically everlasting Ambient temperature operation (no high-temperature corrosion or safety issues) High reliability of solar modules (manufacturers' guarantees over 30 years) Rather predictable annual output Modular (small or large increments) Can be integrated into new or existing building structures Can be very rapidly installed at nearly any point-of-use

Disadvantages of photovoltaics

Fuel source is diffuse (sunlight is a relatively low-density energy) High initial (installed) costs Unpredictable hourly or daily output Lack of economical efficient energy storage

What is the physical basis of PV operation? Solar cells are typically made of semiconductor materials, which have weakly bonded electrons occupying a band of energy called the valence band. When energy exceeding a certain threshold, called the bandgap energy, is applied to a valence electron, the bonds are broken and the electron is somewhat “free” to move around in a new energy band called the conduction band where it can “conduct” electricity through the material1. Thus, the free electrons in the conduction band are separated from the valence band by the bandgap (measured in units of electron volts or eV). This energy needed to free the electron can be supplied by photons, which are particles of light.

Figure 1.2 shows the idealized relation between energy (vertical axis) and the spatial boundaries (horizontal axis). When the solar cell is exposed to sunlight of sufficient energy, the incident solar photons are absorbed by the atoms, breaking the bonds of valence electrons and pumping them up to higher energy in the conduction band. There, a specially made selective contact collects conduction-band electrons and drives these freed electrons to the external circuit. The electrons lose their energy by doing work in the external circuit such as pumping water, spinning a fan, powering a sewing machine motor, a light bulb, or a computer. They are restored to the solar cell by the return loop of the circuit via a second selective contact, which returns them to the valence band with the same energy that they started with. The movement of these electrons in the external circuit and contacts is called the electric current. The potential at which the electrons are delivered to the external world is less than the threshold energy that excited the electrons; that is, the bandgap. It is independent of the energy of the photon that created it (provided its energy is above the threshold). Thus, in a material with a 1 eV bandgap, electrons excited by a 2 eV (red) photon or by a 3 eV (blue) photon will both still have a potential voltage of slightly less than 1 V (i.e. both of the electrons are delivered with an energy of about 1 eV). ...